The overall objective of the research is to develop an understanding of the interactions of laser light with dental hard tissues and to establish a scientific basis for the safe and effective use of lasers for treatment of dental hard tissues including caries prevention and caries removal. There are seven specific aims. The first specific aim is to determine the absorption coefficients for enamel and dentin, with emphasis on the 9.3 and 9.6 um wavelengths which were not able to be fully studied with the lasers available previously. The investigators now have access to a CO2 laser with a shorter pulse duration which will enable measurement of the high absorption at these wavelengths (in vitro). The second specific aim is study the thermal behavior of enamel and dentin follower laser irradiation at selected wavelengths and to model with computer simulations using 1-, 2-, and 3-D finite difference heat conduction numerical codes. This aim improves on their previous 1-D modeling efforts (in vitro). The third specific aim is to study the thermal decomposition kinetics of enamel and dentin during laser irradiation. This aim will utilize six techniques: (1) thermal response analysis based on infrared radiometry measurements of surface temperature, (2) glancing x-ray diffraction, (3) FTIR reflectance spectroscopy, (4) evolved gas analysis, (5) acid dissolution rate measurements, and (6) scanning electron microscopy (in vitro). The fourth specific aim is to carry out safety studies in humans in vivo to establish degree and effects of heat transfer to the pulp during laser irradiation. The study involves 25 volunteers scheduled for removal of 3rd molar teeth in which thermocouple placed at the pulp-dentin junction any temperature rise during laser irradiation (in vivo). Specific Aim 5 is to determine the laser irradiation parameters, with and without fluoride treatments, that optimally inhibit the progression of caries-like lesions in enamel and tooth roots. An acid dissolution assay is used to generate a measure to tooth material hardening by the laser (in vitro). The sixth specific aim is to conduct intra-oral studies in humans involving the placement of dental materials, pretreated in vitro by the laser, into the oral cavity of human volunteers for prolonged periods to study the ability of laser irradiation to harden dental materials. This assay will be conducted through a subcontract with the University of Indiana (in vivo). The seventh specific aim is to study the inhibition of secondary caries around restorations by laser irradiation (in vitro).